The properties of glass make it an ideal substrate material for use in many applications. In particular, the combination of optical transparency, with reasonable strength at a nominal cost, allows the widespread use of glass products. Glass, however, does suffer from several limitations. Glass is not a particularly hard material, and consequently it abrades in many applications. Additionally, glass is chemically reactive with many alkaline substances and with hydrofluoric acid. New applications and superior performance in existing applications could be thus realized for glass products if glass were more abrasion resistant and less chemically reactive. Examples of glass products which could benefit from improved abrasion resistance include eyeglass and sunglass lenses, architectural glass, analytical instrument windows, automotive windshields and laser bar code scanners for use in retail stores and supermarkets.
Diamond-like carbon films (DLC) are well known in the art and have been recognized as potential coatings to enhance the abrasion resistance of various substrate materials, including glass. The DLC coatings possess excellent optical properties and exhibit excellent resistance to abrasion and chemical attack by various acids, including hydrofluoric acid. However, it has been found that the DLC coatings will impart improved abrasion resistance to a substrate only if the adherence of the coating to the parent substrate is excellent.
The most obvious and common approach to coating the glass substrate is to apply the DLC coating directly onto a clean glass surface. However, this approach often results in a DLC coating which displays poor adhesion and therefore, poor abrasion resistance. DLC coatings are typically under significant compressive stress. This stress greatly affects the ability of the coating to remain adherent to the glass substrate. Additionally, glass often contains many alkali oxides and other additives which can inhibit the bonding of the SiO.sub.2 in the glass to the carbon atoms in the DLC coating. It is currently believed that the reaction between the SiO.sub.2 in glass and the DLC is essential for the coating to exhibit excellent adhesion. Therefore, less obvious methods are required to produce a glass substrate with a highly adherent DLC coating which provides excellent abrasion resistance.
In addition to glass substrates, many other optically transparent substrate materials, such as sapphire, glassy-ceramics, salts (NaCl, KBr, KCl, etc.), metal fluorides and metal oxides could benefit from a DLC coating, but contain elements which inhibit the bonding of the DLC layer.
Many methods for depositing DLC have been demonstrated, including radio frequency plasma deposition, ion beam sputter deposition from a carbon target, ion beam sputtered carbon with ion beam assist, direct ion beam deposition, dual ion beam deposition, laser ablation deposition from a carbon target, and ion beam assisted evaporation of carbon. Many of these prior art techniques have been used to deposit DLC on glass substrates, however, the emphasis of the prior art has not been on the adhesion of the DLC to the glass substrate or on the abrasion resistance of the coated substrate product. Illustrative are the following references: U.S. Pat. Nos. 4,746,538; 4,400,410; 4,383,728; 4,504,519; 4,603,082; 4,060,660; 4,877,677; 4,569,738 and 4,661,409; Japanese Patent Nos. 63/221841; 63/221840; 63/195266; 1147068; 1147067; 64-2001; 59-26906 and 51128686; European Patent Nos. DD-203903; SU1006402; Deutchman, et al.; Proc. SPIE-Int. Soc. Opt. Eng. 1146, 124-34, 1989; Collins, et al., Proc. SPIE-Int. Soc. Opt. Eng. 1146, 37-47, 1989; Liou, et al., Proc. PIE-Int. Soc. Opt. Eng. 1146, 12-20, 1989; Bubenzer, et al., Proc. DARPA Workshop Diamond-Like Carbon Coat., Meeting date 1982, Issue AD-A136 766, 33-47, edited by B. Bendow in NBS Spec. Publ. 669, 249-54, 1984; NBS Spec. Publ. 638, 482-82, 1984; Bubenzer, et al., NBS Spec. Publ. 638, 477-81, 1984; Appl. Phys. Lett. 55, 631-3, 1989; J. Vac. Sci. Technol A7, 2307-10, 1989; and D. Nir, Thin Solid Films, 144, 201-9, 1986. These references do not however describe the use of transparent interlayers to improve the adhesion of the amorphous carbon coating to the substrate or substantially optically transparent DLC coatings with greatly improved wear resistance for severe abrasive environments.
The application of low friction coatings, such as tin oxide, aluminum oxide, and boron nitride to optically transparent substrates such as glass is also known in the prior art. However, because these materials have conventionally been applied as thin layers, the wear resistance of the coated substrate in severe abrasive environments has been poor. Conventional tin oxide coatings on architectural glass or glass windows used in supermarket laser bar-code scanners constitute a prior art example of a low friction coating on a transparent glass substrate. In this case, the tin oxide coating (typically 2,000 Angstroms thick) provides an improvement in wear resistance relative to the uncoated glass substrate. However, this coating is neither hard enough, nor thick enough to provide wear resistance in severe abrasive environments. Consequently tin oxide-coated glass windows in supermarket laser bar-code scanners undergo severe abrasion and must be frequently replaced. Aluminum oxide coatings disclosed in European Patent Application #EPO 243541 (WO 87/02713) suffer from the same deficiencies.
Offenlegungsschrift DE 42 01 914 A1, having a United States priority date of Jan. 29, 1991, discloses and claims a scanner window consisting of a transparent substrate, a transparent hard material having a thickness in the range of 500 Angstroms to 10 microns on the substrate, and a transparent slippery top coat comprising DLC, diamond film, polytetrafluoroethylene (PTFE), polyethylene (PE), tin oxide, indium oxide, silicone polymers, boron nitride, aluminum oxide and mixtures thereof deposited on the hard material. Scanner windows that use hard materials in the disclosed lower range do not have sufficient wear and abrasion resistance and those which use the disclosed slippery polymers such as PTFE, PE, silicone polymers and mixtures do not have the desired balance of hardness and slipperiness to compete with those made in accordance with the present invention.